a toolkit for identifying and measuring physical streams features from high resolution dems and...

G EOMORPHOLOGICAL FEATURES EXTRACTION FROM HIGH RESOLUTION DEM S C.Cencetti, P.De Rosa, A.Fredduzzi.

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A toolkit for identifying andA toolkit for identifying andmeasuring physical streams featuresmeasuring physical streams featuresfrom high resolution DEMs andfrom high resolution DEMs anddigital imagedigital image

Authors: C. CencettiP. De RosaA. Fredduzzi

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The problem

● A river is a natural system characterized by continuous changes (in space and in time).

● In most developed countries, over the past decades, the morphology of most rivers have suffered huge changes, mainly due to anthropic interventions.

From Surian & Rinaldi, 2003

The anthropic effects usually cause a narrowing and incision of channel leading to a change of channel type.

Effects:Change in hydrograph response → flooding

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The Challenge

● Stream restoration has emerged as a means to improve aquatic systems by returning streams to their “pre-disturbed state”.

● So hydraulic work or any operation that affect the river should not (or al least minimize) the disturb to the system.

● Became fundamental understand the “equilibrium state” of a river.

– Fluvial geomorphologists are asked to answer the question: is the river/channel in equilibrium or disequilibrium?

Fluvial experts use indices as sinuosity index, braiding index, hydraulic discharge, bankfull limits to support their work comparing this values over time.

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How to compare this indices over time?

Which elevation is should use to derive indices as braiding index (number of channels), flow discharge .

??

Moreover what happens if we want to compare such indices over time remembering that the cross section change in time.

A reference elevation is required

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In the morphological-sediment study of a water course is useful to define a level (and a corresponding flow discharge) ”representative” (and responsible) about the shape and size of river channel. – Formative discharge.

Bankfull discharge

For a river in equilibrium state the formative discharge is the bankfull discharge or better is the discharge where streamflow spills onto the floodplain. [Leopold et al., 1964].

The flows effectively able to erode and shape the channel are not the low discharge (they flows through a channel) and not the highest discharge as they are capable of eroding the channel but they occur so infrequently. It is reasoned, therefore, that exists a range of intermediate discharges that do most of the work of shaping the river channel and that some summary value of these intermediate flows represents the formative discharge of the river (Wolman & Miller, 1960)

The formative discharge corresponds to a discharge with a return period between 1 and 3 years.

Bankfull elevation – bankfull discharge

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formative discharge – bankfull discharge

Bankfull channel ≠ Active channel

Thalweg

● Reconstructing streams to a pre-disturbed state is a difficult task as we know the landscape never return to its condition before human-disturbance

● We rely on accurate determination of the modern bankfull channel geometry to approach stream restoration projects.

Bankfulllimits

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The hub for all indices

#1: Bankfull discharge

Bankfull elevation

#2: Number of channels

#3: Bankfull width

#4: Hydraulic conveyance

The tool here presented is developed to extract such morphological feature directly from an high resolution DEM (LiDAR)

#5: Polygon of channel limits

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bankfull elevation

Bankfull location are identified on the following standard field indicators:● Break in bank slope corresponding with the active floodplain● High-flow markers

➢ Limit of bank scour➢ Roc k staining➢ Sand/silt deposits

● Vegetation limits● Bar tops

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bankfull elevation

The bankfull definition becomes difficult to apply in riverbeds recently carved or still in incision.It may happen that the "active floodplain" is absent and the bankfull discharge could be identified as a discharge where flow spills onto a “terrace”.

This flow may be associated with return periods that excess the 1÷3 years - does not correspond to the formative discharge

The bankfull definition becomes difficult to apply in riverbeds recently carved or still in incision.It may happen that the "active floodplain" is absent and the bankfull discharge could be identified as a discharge where flow spills onto a “terrace”.

This flow may be associated with return periods that excess the 1÷3 years - does not correspond to the formative discharge

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Cross section hydraulic magnitude

Vertical scale: horizontal scale = 10:1hydraulic depth=

wet areawidth

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The methodology

The idea behind the tool is based exclusively on the bank/floodplain slope break.

For each cross section, is graphed the hydraulic depth as a function of flow elevation.

The (local) maximum value of this function indicates a the elevation where water spills across the floodplain

(approach suggested by McKean &Wright 2005 as variant of Williams 1978)

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The tool

The tool developed is able to:

● graph the hydraulic depth vs the elevation

● investigate such function to locate the local maxima and select the first local max corresponding to the first break in slope.

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The methodology – limitation

ISSUE:1)We experienced that the

methodology work better in rivers with floodplain and would be limited in confined channels that miss floodplains.

2)The method is less definitive in channel with multiple terrace, where could be tricky decide which terrace and which plateaux in the hydraulic depth function correspond to the current bankfull elevation

A specific smoothing procedure is required to find the

“significant” local maxima(remove noise)

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The QGIS pluginThe tool have been implemented as QGIS plugin ant it have two main steps:1) Cross sections generation from river axis known longitudinal step and

width2) Evaluation of bankfull elevation (following the proposed methodology) for

each cross section and deriving of all morphological features.

In tab “Numerical setting” the user select the vertical steps and the minimum hydraulic depth.

1

2

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The numerical parameters

Vertical steps: it defines the number of points in function.+ High values are more sensitive for

small slope variations- High computational timeUsually > 500

Minimum hydraulic depth: this setting is to filter out local maxima below such value. Usually: 0.5 m

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The spline smoothing procedure I

A smoothing procedure, based on splines function, is required to remove local maxima not significant (“noise”).

● Explore the range 0÷1 for 100 smoothing parameter (spar).

● To select the best “spar” parameter a k-fold cross validation is used.

● For each “spar” parameter the mean square error and the 1sd is computed

Black dots represent the mean square error and the grey bar the 1sd

The spar selection

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The spline smoothing procedure II

A smoothing procedure, based on splines function, is required to remove local maxima not significant (“noise”).

● spar parameter with minimum error

● The corresponding 1sd

● The highest spar parameter, just smaller that 1sd, is selected

The spar parameter: 1)non significantly

different from the minimum

2)Big enough to smooth data and remove noise

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Evaluation of bankfull elevation

we found the local maxima looking for the zero function of 1st and 2nd derivative of spline using the newthon-rampson method.

The figure shows how The figure shows how this approach is able this approach is able to remove small local to remove small local maxima for a better maxima for a better identification of identification of bankfull elevationbankfull elevation

No selected No selected local maximalocal maxima

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Polygon limits for multiple channels

In case of multiple channel the tool locate the polygon limits to the channel with wet area max.

Channel limits

Banfull width

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The entire workflow

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Case study: Paglia River – central Italy

Reach of 2.2 km of length

XS step: 80mXS width: 400 m

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a

Case study: Paglia River – central Italy

Wandering river in a strong disequilibrium state.● Narrowing and incision

processes● Change in channel

morphology from a braided river to a wandering

● The old active floodplain works now as a morphological terrace

● The active floodplain is now inside the old riverbed, before the narrowing process started.

The blue line is the polygon output of the tool: refers to the limits of the main channel related to the bankfull elevation.

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Results: channel limits in detail

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Local incongruence?

Local incongruence (a) between main channel limits and aerial photo does not appear in the shaded map.

There is a shift in time between LiDAR survey and orthofoto date.

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Result: number of channels vs bankfull width

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 280

50

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1

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bankfull width channels

wid

th (

m)

# o

f ch

an

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ls

In a equilibrium river # of channel is proportional to the bankfull width.

The proportion looks altered.The floodplain is going to became a terrace.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 280

100

200

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1000

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50

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wet area

Bankfull di-scharge

m3

/s

Are

a (

m2

)

Result: wet area and bankfull discharge

A good correlation between discharge and wet area.

1)For some cross section the proportion is altered (see 13÷14 and 26÷27) – low bed slope

2)Section 1 has very higher value of area → high discharge

Q =χ⋅S⋅√R j

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Cross section #1

● The cross section look carved and in this point the river is partially confined (the floodplain in right is missing).

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Result: discharge variation and water table

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28-2

0

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maxWTWT linear egressionvariation

As the discharge may be affected by global error, as the DTM LiDAR does not include the under-water, we should be more interested in variation rather that the discharge itself.

Potential pitfallXS # 26 → Increasing of cross section area.XS #1 → carved cross section.

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Conclusions

● Advantages

– The tool/plugin is designed to be extremely simple to use (a DEM input and two numerical parameters).

– The tool use a robust methodological approach.

– The bankfull limits are correctly located compared to aerial photo.

– Several parameters useful to evaluate the “state” of a river.

– Few tool that extract river morphological features exists already implemented in a GIS environment

● Disadvantages

– The methodology base the location of bankfull limits only on the slope/break in slope. Some other collateral indicators, from aerial photo (?), using image classification procedure could/should be inserted.

– The tool is less definitive for river that miss floodplain, or in strong disequilibrium, or for braided rivers.

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THANKS FOR YOUR ATTENTIONTHANKS FOR YOUR ATTENTION

The authors acknowledge Luca Scrucca for the collaboration in the The authors acknowledge Luca Scrucca for the collaboration in the the statistical smoothing procedure.the statistical smoothing procedure.

Plugin available at:https://github.com/pierluigiderosa/BankFullDetection

a toolkit for identifying and measuring physical streams features from high resolution dems and...

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